"An overall guaranteed income program ... worthy of consideration (must) offers a substantial level of benefit to people who are normally in the labour market. Therefore, a great deal of further study and investigation, like the experiments now under way in New Jersey and Seattle in the United States, is needed to find out what effects such a program would have on people's motivation, on their incentives to work and save. Until these questions are answered, the fear of its impact on productivity will be the main deterrent to the introduction of a general overall guaranteed income plan. "
You will find that this article concludes that a GAI has no negative effect on the incentive to find work.
"Moreover, while directly improving the standard of living of their target populations, they would do so while encouraging rather than dampening incentives to become more self-sufficient through earnings."